1. Field of the Invention
This invention relates to optical instruments, and particularly to a Berek compensator having a birefringent crystal, which can be both tilted and rotated by means of adjustment rings which are concentric to the optical path of the device.
2. Description of the Prior Art
In the field of experimental optics there is a frequent need to introduce a variable phase difference in an optical beam. Various devices exist for this purpose, perhaps the best known of which is the Soleil-Babinet compensator. The Soleil-Babinet compensator utilizes a pair of rectangular-shaped crystal elements having their optical axis perpendicular to each other so that the ordinary ray in one is the extraordinary wave in the other. One of the rectangular elements is further divided into two wedge-shaped sections. One of the wedge sections is movable with respect to the other so that the total length of the optical path through the pair of wedges is variable with respect to the length of the optical path in the undivided rectangular element. Thus, the phase shift of the incident ray is proportional to the relative lengths of the optical paths. Such devices are described in Principles of Optics, 6th Edition, Max Born and Emil Wolf at pages 693-694, and are commercially available for example from Melles Griot, Inc., Irvine, Calif. Another type of variable retarder is described in U.S. Pat. No. 3,924,930, and is commercially available from Cleveland Crystals, Inc., Cleveland, Ohio.
While the Soleil-Babinet compensator can provide the requisite optical function of a variable phase shift, it requires the fabrication of two wedge-shaped crystals. It also requires a mechanical mechanism to move one wedge relative to the other while maintaining all elements in precise optical alignment. The device also suffers from the requirement for motion transverse to the optical path, thereby adding to the size of the device. Size is not a trivial aspect since it would be desirable to have a compensator having minimal diameter which would permit mounting in a universal type of optical mount of the type having adjustments facilitating the initial set-up and alignment. Similarly, other commercially available retarders require complex optical and mechanical element fabrication, and are large compared to the usable optical aperture.
Another variable phase shift device, known as the Berek compensator, is described in Principles of Optics at page 694. In this device, the active element is a single crystal of birefringent material positioned so that the optical axis is perpendicular to the parallel faces of the crystal. The variable phase shift is created by tilting the crystal relative to the incident beam. While the Berek compensator is well understood and widely known, it has not found wide application in the laboratory.
The lack of practical application of the Berek compensator may be due, at least in part, to the requirement for tilting the crystal and the attendant mechanical complexity necessary to provide the requisite precision control.